1. Field of Invention
The invention relates to a magnetiser for a pipeline inspection tool.
2. Description of Related Art
Pipelines carrying liquid or gaseous products can be inspected automatically from the inside using devices known as intelligent pigs. These devices are usually propelled down the pipe under the flow of product in the pipeline, and utilise magnetic, ultrasonic or other non-destructive techniques to inspect the condition of the pipe wall.
For magnetic methods of inspection, known as the MFL (Magnetic Flux Leakage) technique, the pig has permanent magnets defining first and second pole pieces, which are positioned adjacent the inner wall of the pipe. Those magnets generate magnetic fields which magnetise the wall of the pipe. The MFL technique operates on the principle that an unusual feature in the pipe wall (such as a defect, weld bead or wall thickness change) will disrupt any magnetic flux applied therein, and this disruption (or leakage) may be detected, e.g. by sensors provided between the magnetic poles that detect the magnetic flux density at the internal surface of the pipe. As the pipeline pig is driven along the pipe, the location of the pole pieces, and the sensors, moves along the pipe enabling the internal surface of the pipe to be inspected.
FIG. 1 shows a schematic representation of a magnetising assembly 100 for a conventional MFL inspection device. The assembly 100 comprises a central body 1 of mild steel or other ferromagnetic material extending in an axial direction, with radially magnetised magnets 2 fixed around each end to form an annulus. The polarisations, or directions of magnetisation (DOM), of the magnets at each end are opposite to each other, as indicated by respective arrows 12, 13. When the device is inserted into a pipe, ferromagnetic flux couplers 3 couple the flux from the magnets 2 to the pipe wall. The flux couplers 3 may be flexible or semi-rigid, e.g. mild steel bristles. They may provide suspension for the device, e.g. to maintain centrality in the pipe. A ferromagnetic mounting plate or ring 4 allows for easy replacement of the flux couplers 3. The magnets 2 are each protected by steel plates 5, 6 and 7, which form an enclosure to provide protection for the magnets. To allow magnetic coupling between the magnet 2 and the mounting plate 4, the top plate 5 is made from a ferromagnetic material, and to prevent shorting of the magnet 2, the side plates 6 and 7 are made from non-magnetic material.
The arrangement shown in FIG. 1 is rotationally symmetric about the body's axis A-B to impart a uniform axial (along the pipe) magnetisation in the pipe wall, permitting inspection by the MFL method. FIG. 2 shows a typical contour plot of lines of magnetic potential from such an arrangement inside a pipe. Here it can be seen that a magnetic flux circuit is generated, whose path flows between the magnets 2 through the pipe wall and central body 1. The central body 1 is often referred to as the return path, because of its role in providing a return route for the magnetic flux circuit between the magnets.
In one example of an inspection device, a circular array of sensors (not shown) would be mounted on the body between the annular magnets in contact with the pipe wall. In the presence of a defect some of the magnetic flux leaks out of the pipe wall and is detected by one or several of the sensors.
An example of the above described scheme is disclosed in U.S. Pat. No. 4,447,777. Other examples, which may be used where the pipeline is straight, with gentle or no bends, the magnets may be shaped, and even non-contacting with the pipe wall (see e.g. U.S. Pat. No. 6,198,277).
The arrangement shown in FIG. 1 is just one possible example of the MFL technique. In another example, the central body may be divided into axially extending segments and mounted off a central suspension mechanism. This obviates the need for the flexible flux couplers of the previous example, allowing shorter, less flexible members to be used to provide the magnetic coupling instead. Examples of such arrangements can be found in U.S. Pat. No. 4,105,972, U.S. Pat. No. 4,310,796, U.S. Pat. No. 5,864,232 and U.S. Pat. No. 6,762,602. All these examples rely on a magnetising assembly having oppositely polarised magnets mounted on both ends of a ferromagnetic magnet bar or return path.
In addition to MFL inspection using an axial magnetic field, transverse field inspection can be achieved by magnetising the pipe wall in the circumferential (around the pipe) direction.
At its most general, the present invention proposes mounting an additional permanent magnet on a magnetising assembly for an in-line pipe inspection tool to enhance (e.g. increase the density of) the magnetic flux in the pipe wall compared with conventional tools of the same size. An increased flux density may permit the tool to perform reliable inspection of thicker pipe walls, smaller pipe diameters and multi-diameter pipeline networks.